Select random task in task manager, open it in JOSM, select preferred Imagery.

Switch to tennis-outdoor.osm layer (click on the 'active' column in Layers window), select the court node in the center and copy all of its tags to clipboard.

Switch back to OSM Data Layer and trace the tennis court area if it already doesn't exist there. Create multipolygon if courts cannot be traced as one shape. Do NOT trace indoor courts, even if they are part of the complex.

Paste tags from the clipboard to the courts shape. Map and connect any missing OSM elements nearby, such as footways, playgrounds, service ways etc. If the court is painted on an ice rink copy-paste entire node in the middle of the court.

Upload changes to OSM. Make sure you do this with OSM layer being active (otherwise JOSM won't allow you to).